Lasing in direct energy gap semiconductors has been well known since the 1960s when low-temperature GaAs homojunctions lasers and subsequently room temperature AlGaAs—GaAs double heterojunction lasers were first reported. This was followed by the development of quantum well, quantum wire and more recently, quantum dot lasers. Both edge-emitting and surface-emitting configurations are also well known in the prior art. Semiconductor lasers are fabricated using direct gap materials active layers such as GaAs, InGaAs, InGaAsP, ZnCdSe, and GaN. Unfortunately, this precludes using widely used materials such as Si (the mainstay of integrated circuit industry), Ge, GaP, and other compound semiconductors for fabrication. While most of the semiconductor lasers were reported in inorganic semiconductors, organic semiconductor lasers have also been reported.
It should be appreciated that light emission in indirect energy gap semiconductors, such as Silicon, gallium phosphide (GaP) and other indirect energy gap materials, exhibit significantly lower quantum efficiency ηq and, hence, optical gain when p-n junctions are configured as lasers and result in impractical threshold current density values. In the case of indirect gap semiconductors, such as Si and Ge, on calculation determined that the free carrier absorption is considerably higher than the gain produced by the current injection. Recently, lasing in Ge film grown on Si is also reported with threshold current density over 100 kA/cm2. Additionally, Si Raman lasers operating at photon energies below the band gap have been reported using optical pumping. Tensile strained Ge lasers are also proposed. Unfortunately, the limitations associated with band-filling and tensile-strained Ge structures have not resulted in lasers with acceptable threshold current densities from practical on-chip integration perspective. Excitonic formation in GaP:N or GaAsP:N layers is well known to result in efficient light emission in p-n junctions (when the composition makes the band gap to be indirect, i.e. phosphorus in excess of 44%). However, the threshold current density is very high in this type of system, and as a result one has to operate at lower temperatures (below 77K) to obtain stimulated emission. Another way of looking at it is that the binding energy of excitons in bulk is about 4 meV. The threshold current density and operating temperature can be improved by enhancing the exciton binding energy. No reports are available which describes significant formation of excitons (and eventual lasing) without the doping of isoelectronic impurities such as nitrogen in GaP.
The optical gain has been shown to increase in active layers realized in indirect gap materials by introducing shallow iso-electronic impurities such as N and Bi in GaP or GaAsP. For these impurities, the electron states are highly localized forming bound excitons. The electronic state is primarily made of conduction band Bloch functions, relevant to the central region of the Brillouin zone. The electron-hole pairs, forming the excitons, recombine via vertical transitions, yielding low radiative lifetime τr and high internal quantum efficiency ηq. It has been demonstrated that the excitonic transition corresponding to A-line in InGaP:N can give lasing when the composition of the active layer is such that its energy gap crosses over slightly to the indirect side. Quantum Confined Stark Effect (QCSE), which depends on the existence of excitons, has not been observed in type-I SiGe/Si multiple quantum wells (MQWs). This is attributed to the lack of appropriate quantum well barrier heights (or offsets) for electrons (ΔEc) and holes (ΔEv) in the conduction and valence band, respectively. In the case of Si—Ge system, the magnitude of conduction and valence band offsets can be increased if type II heterojunctions are used. It has been reported that layering structure can be used to obtain these offsets. Active layers comprising of type-II strained-layer SiGe/Si quantum wells, wires and dots are described for obtaining lasing. Recently, lasing has been reported in Ge layers grown on Si at threshold current density of about 300KA/cm2. This threshold permits use as pulsed laser. In comparison to direct lasers, the threshold current density is about 3 orders of magnitude higher.